This invention concerns a time domain reflectometer for making impedance measurements on a transmission line cable system in the powered state.
High-fidelity transmission on an electromagnetic transmission line requires a lack of major impedance discontinuities. Thus, although transmission lines necessarily include bends, twists, joints, connectors and the like, the resulting impedance discontinuities caused thereby, as well as those caused by line damage, should be kept to a minimum. Transmission lines are periodically tested electrically in oroer to determine the nature, location, and amplitude of impedance discontinuities therealong and to determine whether these discontinuities are within desired limits.
Time domain reflectometry (TDR) is a well-known method for determining the general characteristics of impedance variations in a transmission line . In this method a test pulse or step waveiorm is transmitted down the line and the reflection from an impedance discontinuity is detected together with the time it takes for the pulse to reach the discontinuity and return. The location of the discontinuity is determined by observation of the elapsed time between the transmitted pulse and the reflected pulse. This technique is highly sensitive, revealing not only gross defects, such as open or short circuited cables ano terminations, but also revealing quite minute variations, e.g., cable impedance variations, frayed shields, and impedances introduced by making tap connections to the cable.
One prior art time domain reflectometry system is described in U.S. Pat. No. 3,434,049 to Frye. Frye discloses a current pulse source for producing a step wave voltage across the impedance of a transmission line which current source, because of its high output impedance, produces substantially no loading effect on the line and is therefore capable of driving lines of widely different characteristic impedance. A tunnel diode oscillator is used for delivering a rapid rise time current pulse to the transmission line with a minimum of circuitry.
Ethernet (trademark of the Xerox Corporation) cable systems have stringent requirements on cable impedance uniformity, termination matching, and maximum capacitance introduced at a transceiver tap. In addition, it is preferable to make TDR measurements on an Ethernet system which is powered up, since the impedance discontinuity introduced at a transceiver tap could differ between the powered and unpowered states.
Testing a powered Ethernet system imposes special constraints on the TDR apparatus to be used for measurements. FIG. 2 shows the pertinent voltages encountered on an Ethernet cable. Normal voltage limits for the transceiver attachment to the center conductor are 0 V (positive rail) and -6 V (negative rail). Most transceiver designs can probably tolerate applied test voltages outside these rail limits for some brief time, but the voltage excess and time tolerance are highly dependant upon individual transceiver designs. Furthermore, even if the transceiver can tolerate excess voltages, the tap impedance may be altered if the applied voltage exceeds the rail voltages by more than a diode drop. Conversely, measurements made within the limits of the rail voltages plus a diode drop shoulo represent the true tap impedance.
The collision detection band of an Ethernet system lies from -1.40 V to -1.65 V. The average voltage from transmission by a single transceiver always lies above this band. If two transceivers are transmitting simultaneously, the average voltage from the overlapping pulsed transmissions will lie below the collision detection threshold of -1.40 V. When a transceiver detects an average voltage level below the collision detection threshold, the transceiver will cease transmission and retry after the collision voltage disappears. If an Ethernet cable system is to be tested in the powered state while the transceivers are transmitting information signals, one must avoid using a test pulse which in combination with a transmitted signal being received by a transceiver causes the average voltage level on the center conductor to drop below the collision detection threshold, thereby causing rejection of the received signal by the transceiver.
In general, positive test pulses greater than a few tenths of a volt cannot be used for testing a powered Ethernet cable system because the tap impedance measurements are not reliable when the voltage test pulse exceeds the upper rail margin, and furthermore, transceiver damage can result.
Therefore, it is an object of the present invention to provide an improved time domain reflectometer which permits impedance measurements to be made on a transmission line cable system in the powered state.
It is another object of this invention to provide a time domain reflectometer which generates negative test pulses wherein any positive reflections do not exceed the upper rail tolerance.
A still further object of this invention is to provide a time domain reflectometer which applies a negative DC bias voltage along with a negative test pulse to provide additional headroom for positive pulse reflections. Furthermore, the bias voltage conveniently allows all transceivers to be disabled from the TDR test site rather than at each interface site, wherein the disabling of the transceivers prevents network activity from obscuring the TDR measurements.